The heterojunction bipolar transistor (abbreviated as HBT hereinafter) is a type of bipolar junction transistor (BJT) that uses different semiconductor materials for forming a heterojunction between emitter and base therein. The advantage of HBT is the high current gain and the low base resistance. In addition, the HBT made of compound semiconductor layers epitaxially grown on a GaAs substrate (abbreviated GaAs HBT hereinafter) has the high electron mobility due to the material properties of those layers, and is of great advantage to high-frequency applications. For example, the GaAs HBT is commonly used in cellular phone handsets, WiFi terminals, and their base stations as RF power amplifiers and other monolithic microwave integrated circuits (MMICs). The performance of the GaAs HBT is significantly improved by band gap engineering of the base, the emitter, and/or the collector using strained (pseudomorphic) semiconductor layers or semiconductor layers with graded composition. The conduction electron transit time of the HBT is thereby reduced leading to the improvement of high-frequency performance such as the higher current gain cutoff frequency (fT hereinafter) and the maximum oscillation frequency (fmax hereinafter).
A double HBT (abbreviated as DHBT hereinafter) is another type of HBT in which the band gap of both emitter and collector is wider than that of the base. It is known that an InGaP/GaAs/AlGaAs DHBT formed on a GaAs substrate can reduce the base-to-collector capacitance Cbc in the bias region of low voltage drop across collector and emitter (Vce) and high collector current (Ic) (i.e. knee region). The reduction of Cbc leads to the high linearity in a power amplifier, such as the low error vector magnitude (EVM) of a WiFi signal modulation (refer to 13th GAAS Symposium, Paris, 2005, pp. 205-208). It is explained that the Cbc reduction is attributable to the hole blocking by the valence band offset at the base-collector junction. Since the bandgap of the collector is larger than that of the base, there is a valence band offset at the base-collector junction in DHBTs, which blocks the diffusion of holes into the collector, and hence reduces the diffusion capacitance. However, IT and fmax of the InGaP/GaAs/AlGaAs DHBT are 30 GHz and 57 GHz, respectively, which are smaller than those of a typical GaAs HBT, i.e., more than 40 GHz in IT and more than 100 GHz in fmax. The IT and fmax of the HBT are related to the mobility of electrons in the material used for forming the base and to the saturation electron velocity of the material used for forming the collector. In order to improve the operation speed of DHBT, indispensable is to use a material with higher electron mobility for forming the base and a material with higher saturation electron velocity for forming the collector.